The terrestrial positioning of cellular telephone handsets in cellular communications networks with sufficient accuracy to support enhanced emergency 911 (E-911) services will soon be mandatory in the United States of America. Non-regulatory, market driven forces are also motivating communications equipment manufacturers and service providers to more accurately locate cellular telephone handsets and other mobile stations, for example to provide promotional and fee based value-added services.
Network based schemes for locating cellular handsets in cellular communications networks are known generally, for example those based on signals received at the mobile station from several neighboring cellular base stations as the mobile station moves about the cellular communications network. Known network based mobile station location determinations include, among others, those based on signal strength measurements, or on Angle of Arrival (AOA) measurements, or on Time of Arrival (TOA) measurements, or on Enhanced Observed Time of Arrival (EOTD) measurements, or on Time Difference of Arrival (TDOA) measurements, or on Advanced Forward Link Triangulation (AFLT) measurements, among others.
Network based mobile station location determinations however are subject to inaccuracies and positioning error. In some networks, for example, handset power limitations and the inability of the handset to maintain constant communication with a sufficient number base stations, among other factors, limits the accuracy of network based location determinations. See generally, U.S. Pat. No. 6,081,229 entitled “System and Method For Determining The Position Of A Wireless CDMA Transceiver”.
Another approach to determining the location or position of cellular handsets is the use of satellite based positioning systems, which are generally capable of providing longitude, latitude, altitude and velocity information about the mobile station, although in some cellular handset positioning applications only longitude and latitude coordinates are required. Satellite based terrestrial position determinations are made generally by acquiring satellite data, with a satellite receiver integrated in the mobile station, from several satellites among a constellation of satellites. Known satellite positioning systems include the Navigation System with Time and Range (NAVSTAR) Global Positioning System (GPS) in the United States of America, and the Global Orbiting Navigation System (GLONASS) in Russia.
In cellular communications network applications, it is also known to generate satellite based location fixes with assistance from the communications network. See, for example, U.S. Pat. No. 6,058,338 entitled “Method and Apparatus For Efficient GPS Assistance In A Communications System” and U.S. Pat. No. 5,982,324 entitled “Combining GPS With TOA/TDOA Of Cellular Signals To Locate Terminal”.
Satellite positioning system based position determinations are also subject to inaccuracies and positioning error. In cellular and other mobile station positioning applications, mobile station location error may result from obstruction of satellite and any terrestrial based assistance signals, for example by obstacles in urban canyons, and from signal propagation delay, among other factors.